FIG. 9 is a sectional view outlining the arrangement of a conventional gas laser device disclosed by Japanese Patent Application (OPI) No. 192285/1988 (the term "OPI" as used herein means an "unexamined published application"). FIG. 10 is a plan view outlining a resonator in the laser device. In these figures, reference numeral 11 designates a 72 MHz high frequency generator; 21, a power matching circuit; 22, a high frequency cable; 23, an insulating feed-through; 71 and 72, electrodes; 73 and 74, the surfaces of the electrodes, which are polished into optical reflecting surfaces; 75, a discharge gap; 76 and 77, spacers insulating the electrodes 71 and 72 from each other; and 78, a U-shaped base. The electrodes 71 and 72 with the spacers 76 and 77 are mounted on the base 78, and the latter 78 is closed with a cover 79 in such a manner that a ceramic insulator is disposed between the cover 79 and the electrode 71. The laser resonator, as shown in FIG. 10, is made up of a total reflection mirror 52 having a concave surface and a total reflection mirror 53 having a convex surface.
In the conventional gas laser device thus constructed, the high frequency generated by the high frequency generator 11 is applied through the power matching circuit 21 and the cable 22 to the discharge gap 75 between the electrodes 71 and 72. The discharge gap is filled with laser gas, and therefore the laser gas is discharge-excited by the high frequency thus applied. That is, the laser gas thus excited is present in the laser resonator comprising the reflection mirrors 52 and 53, so that a laser oscillation is carried out. In this operation, a square beam about 2.times.2 mm.sup.2 can be obtained by setting the distance between the electrodes 71 and 72 to 2 mm and by setting the distances between the edges of the electrodes 71 and 72 and the edge of the convex mirror 53 to 2 mm. This square beam is converted into a Gaussian circular beam at a certain distance from the laser resonator.
In the above-described conventional gas laser device, its laser resonator is a so-called "positive branch unstable resonator comprising a concave mirror and a convex mirror. Therefore, the resonator is considerably sensitive to the inclination of the reflection mirrors; that is, it is difficult to adjust the inclination of the reflection mirrors. And the adjustment is liable to change for instance because the mirrors are deformed as their temperature changes. Thus, the conventional gas laser device suffers from a difficulty that the resonator is poor in stability.